This is the factory image from Google made into a flashable zip with CWM or TWRP.
Clear cache/dalvik cache but no need to clear your user data.
Warning: CWM users have reported that the "Disable Recovery Flash" option is breaking root. Be advised to choose "No." I do not have CWM so I cannot confirm but there is no reason to disable the recovery flash because no recovery is included in the zips.

If flashing from CM10.1 or some other custom roms, you may need to clear the Clock app's data.

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